call path.basename w/ extension name to avoid e.g. `.js.1.js`

lib/core/fs.js

@@ -120,9 +120,10 @@ function copyPreserve(sourceFilePath, targetFilePath) {
   let ext = 1;
   let preserved = targetFilePath;
   while (fs.existsSync(preserved)) {
+    let extname = path.extname(targetFilePath);
     preserved = utils.joinPath(
       path.dirname(targetFilePath),
-      `${path.basename(targetFilePath)}.${ext}${path.extname(targetFilePath)}`
+      `${path.basename(targetFilePath, extname)}.${ext}${extname}`
     );
     ext++;
   }